A Dual Isotope Conundrum: Unraveling a Cryptic Subsurface N Cycle

氮是一种重要的常量营养素，在很大程度上调节着全球生态系统的初级生产力。由于人类活动，全球氮的周转量增加了一倍以上，导致生态系统基本动态发生了大规模和广泛的变化，对人类和生态健康都产生了显著影响。因此，准确了解氮的（生物）地球化学循环对于预测其在环境中的命运至关重要。本项目旨在通过全面考察NO3-（和NO2-）与Fe之间的生物和非生物相互作用及其对环境中NO3-的双同位素组成（如δ 15N和δ 18O）的影响，量化地下陆地生物圈中N和Fe循环之间的联系。具体而言，基于微生物培养的反硝化同位素研究与基于野外的厌氧条件下地下水NO3-损失同位素测量（其中反硝化作用广泛涉及）之间存在N和O同位素耦合效应的根本差异。我们认为，这种差异是普遍存在的，但在很大程度上未被认识到的，非生物和/或生物铁循环与还原性和厌氧氧化N循环途径耦合的结果，并有初步数据支持这一假设。对这一假设的验证将有助于揭示这一明显同位素难题的潜在原因，并将对提高我们对地球地下N、C和铁循环耦合的理解具有重要意义。该项目还将支持开展为期两天的年度教师培训研讨会，以支持马萨诸塞州的科学、技术、工程和数学教育。我们将与瓦奎特湾国家河口研究保护区（WBNERR）合作，举办一个研讨会（约15名高中教师参加），重点讨论流域的废水问题和沿海生态系统的氮循环。讲习班将使与会者了解自然氮循环、人类影响和环境影响，以及尽量减少人为氮负荷对当地流域和脆弱沿海生态系统影响的方法。

Nitrogen is an essential macronutrient that largely regulates primary productivity across a host of global ecosystems. Through human activities, global turnover of N has more than doubled, resulting in massive and widespread alteration of fundamental ecosystem dynamics leading to pronounced impacts on both human and ecological health. An accurate understanding of the (bio)geochemical cycling of N is, therefore, of central importance to enabling the prediction of its fate in the environment. This project aims to quantify linkages between the N and Fe cycles in the subsurface terrestrial biosphere, through a comprehensive examination of the biotic and abiotic interactions between NO3- (and NO2-) and Fe and their influence on the dual isotopic composition (e.g., delta 15N and delta 18O) of NO3- in the environment. Specifically, there exists a fundamental discrepancy of coupled N and O isotope effects between microbial culture-based isotopic studies of denitrification and field-based isotopic measurements of groundwater NO3- loss under anaerobic conditions (in which denitrification is widely implicated). We suggest that this discrepancy is the result of a ubiquitous, yet largely unrecognized, coupling of abiotic and/or biotic Fe cycling with both reductive and anaerobic oxidative N cycling pathways and have preliminary data supporting this hypothesis. Testing of this hypothesis will help reveal the underlying causes of this apparent isotope conundrum and will have important implications for improving our understanding of the coupling of subsurface N, C and Fe cycling in the terrestrial subsurface. This project will also support the development of an annual 2-day teacher-training workshop in support of Science, Technology, Engineering and Math education in Massachusetts. We will partner with the Waquoit Bay National Estuarine Research Reserve (WBNERR) to conduct a workshop (about 15 high school teachers) focused on wastewater issues in watersheds and nitrogen cycling in coastal ecosystems. The workshop will provide participants with an understanding of the natural nitrogen cycle, human impacts and environmental effects and approaches for minimizing the impact of anthropogenic N loading in local watersheds and fragile coastal ecosystems.